Light-driven Molecular Magnetic Switch for a Metallofullerene

Metallofullerene Sc3C2@C-80 was synthesized by the arc-discharging method and isolated by multi-stage high performance liquid chromatography. Two Sc3C2@C-80 azobenzene nitroxide radical derivatives, compound I and II were synthesized through a Prato reaction, respectively. Usually, azobenzene undergoes trans-cis isomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis-trans isomerization can be driven by light or occurs thermally in the dark. Thus, the compound I was excited by UV light, and it exhibited a strong absorption band decrease at around 340 nm and a slight increase at about 470 nm, which belonging to the pi ->pi* and n ->pi* transition respectively. Similar to the azobenzene molecule in solution, the typical change of the absorption spectrum of compound I can be ascribed to the trans-to-cis transition with UV light irradiation. Similarly, the reverse isomerization of UV-irradiated compound I with visible light (475 nm) resulted in an obvious pi ->pi* band increase, also indicating the reverse isomerization of compound I from cis-to-trans form. Besides, the structure and spin density distributions of compound I were calculated as well. It has two unpaired spins localizing on the Sc3C2@C-80 moiety and nitroxide radical. The magnetic property of metallofullerene can be manipulated by the spin interactions of the two spin centers. The electron paramagnetic resonance (EPR) signals of the trans isomer of the compound I are almost independent of each other. After UV light irradiation, the distance of the two spin centers decreased to r=0.752 nm, and the strong spin-spin interaction weakened the EPR signals of Sc3C2@C-80. However, the decreased chain length between Sc3C2@C-80 and nitroxide radical would result in a weakened spin-lattice interaction, which increased the EPR signals of the nitroxide radical. Moreover, the UV-radiated compound I with visible light treated later for several minutes, and the EPR signals of Sc3C2@C-80 has a certain degree of recovery with visible light irradiation. Therefore, the compound I has sensitive and reversible spin variation with different light irradiation. The remote nitroxide radical group serves as a magnetic switch for the EPR signal of Sc3C2@C-80 through the photoisomerization properties of azobenzene bridge. The EPR signals of Sc3C2@C-80 moiety were decreased by the strong spin-spin interaction, and the EPR signals of Sc3C2@C-80 would be enhanced by larger space with visible light irradiation. Such magnetic switch for metallofullerenes has potential applications in quantum information processing and molecular devices.

Automated summary

Metallofullerene Sc3C2@C-80 was synthesized and isolated. Two azobenzene nitroxide radical derivatives, compound I and II, were synthesized through a Prato reaction. Compound I exhibits sensitive and reversible spin variation and the remote nitroxide radical group acts as a magnetic switch.